U.S. Pat. No. 2,843,801 has disclosed the efficacy of hafnium oxide as an electron emitting material and U.S. Pat. No. 4,044,276 discloses emitter materials composed of barium oxide, calcium oxide and hafnium oxide. Attempts to employ the latter material, while successful, have problems due to moisture reaction. This material was coated on electrodes as the carbonates and oxides and subsequently fired in hydrogen or vacuum to decompose the carbonates to the oxides. However, after firing, it has been discovered that the materials can deteriorate rapidly when exposed to ambient atmosphere. Calcium oxide can react vigorously and exothermally with water vapor while the calcining of barium carbonate is an equilibrium reaction, thus: EQU BaCO.sub.3 .revreaction.BaO+CO.sub.2
and, while this reaction can be driven to the right, exposure to CO.sub.2 after firing can result in the re-formation of BaCO.sub.3.